Arctic sea ice data sets in the context of climate change during the 20th century

Available estimates of sea ice extent in the northern hemisphere cover the period from the early part of the 20th century to present day. We analyze changes in ice extent and thickness in the Arctic and its relation to surface air temperature over this period. Time series obtained from different data sets demonstrate better agreement after the 1950s and especially since 1979 with the onset of regular remote sensing observations from satellites. Statistics of time series show minima ice extent in August-September. Mean square deviations reach maxima in July-August. The distributions of trend coefficients show a more significant decrease of summer ice extent. Statistics of monthly ice extent in the Siberian Arctic seas show a similar distribution. September ice extent in the majority of the Siberian Arctic seas and in the Barents Sea reveal rapid shrinking during Arctic warmings in the 1920-1940s and 1990s. Significant correlation between surface air temperature and ice extent occurs in summer months with maximum in June under the influence of June maximum solar irradiation, and amplified by heat advection in the atmosphere and ice extent anomalies in the previous months. The relationship between variations of winter air temperature and ice extent is weaker because winter ice extent anomalies depend on air temperature anomalies as well as on the area occupied by a freshened upper layer. Good agreement between variations of the sum of summer air temperature in the marine Arctic and sea ice extent in September is found (correlation coefficient is 0.85). It confirms that summer melting plays the most important role in the sea ice volume decrease. The renewed observations in 2004-2005 at the Russian "North Pole" drifting stations revealed that the area-averaged perennial ice in the Arctic Basin decreased by 110 cm relative to the 1990 value. But the land-fast ice thickness in the Kara and Laptev Seas show an insignificant positive linear trend for 1934-2005 in agreement with the sum of winter air temperature. The negative trend of land-fast ice thickness becomes apparent starting from the 1970s.